It’s worth noting that Twitter’s apparent success is something of a mirage. People tend to think that Twitter has a lot of users. But if you look closely, the illusion falls apart. Millions of people have signed up for the service but have never used it. Millions more used it for a while, but stopped.

For example, out of 175 million registered accounts, only 119 million were actually following someone in April. If you don’t follow on Twitter, you don’t see any posts. Only 85 million accounts had one or more followers. If nobody follows you, you’re not communicating with anyone. You’re not really a “user” if you’re not using the service.

Twitter defines an “active user” as one who follows at least 30 people and has at least 10 people who follow him. A source with access to Twitter’s API who was quoted by Business Insider in April says that there were only 21 million people or accounts on Twitter that met the “active user” criteria.

Google+ probably has more than 21 million users already, although numbers on how many of those are “active” have not been published. In other words, the number of users Google+ has gained in three weeks is equal to the number of active users Twitter has gained in five years.

Twitter is extraordinarily vulnerable, especially since Google+ users are exactly the same kinds of people who will want to use Google+: Pundits, celebrities, business owners, bloggers and people involved in politics. In addition, Google+ will appeal to the kinds of users turned off by Twitter: teens, Facebook refugees, and the masses of people who don’t want to learn Twitter’s unique slang and command codes.

I believe a majority of Twitter’s most active users will also be on Google+ within a few months after Google opens its social networking site to the public. But most of the active users of Google+ will not be on Twitter.

I also predict that a growing number of the links on Twitter will direct followers to original posts on Google+, where a real conversation can take place.

Increasingly, Twitter will become an empty shell, a place where most of the posts are placed there automatically from the services where people are genuinely active, and where many of the links will take followers outside of Twitter to social sites like Google+ for ensuing conversation.

Celebrities will prefer G+ because its pictures, videos and viral sharing will give them better control over their images and because it will give them access to a bigger potential audience for their posts.

Pundits will like G+ better because it’s better for crowdsourcing and feedback. (I crowdsourced ideas for this column on Google+ and got high-quality input from 100 people in three hours.)

Bloggers will like it because Google+ is the most friction-free blogging platform, with most of the social attributes of Tumblr. Several prominent bloggers have already closed their existing blogs and now just use Google+. In the future, many bloggers will feed their blogs into Google+, or will feed their public Google+ posts out of that service and into their custom-designed blogs.

Today, Twitter still has a lot of fans and defenders, even on Google+. Twitter is currently a better megaphone than Google+; it’s better for talking at a large audience without having them engage you back. It’s much better today for quick news because all the news sources have established feeds on Twitter. It’s easier to skim, unlike Google+, which is wordy and time-consuming. And Twitter allows anonymity, which is better for people who want to criticize repressive governments.

But that’s today. Tomorrow, most or all of those advantages will be erased by improvements to Google+, the addition of third-party add-ons and apps, and the participation of businesses, publications and a lot more people.

I don’t see how Twitter can defend itself from the Google+ challenge.

When you add up what Google+ can do today and what it will do tomorrow, it’s clear that Twitter is perfectly obsolete.